This invention deals generally with refrigeration, and more specifically with metal hydride heat pumps powered by waste or solar heat.
Metal hydride technologies have been well established for nearly two decades. Hydrogen reacts with metals to form metal hydrides, a reaction that is reversible depending upon the temperature and pressure. Metal hydride heat pump technology is receiving increasing attention in recent years, partially due to the call for more compact and higher performance thermal control systems that consume minimal electricity. A metal hydride heat pump uses the energy associated with the hydrogen and metal reaction to perform heating and cooling, and thereby consumes minimal electricity. Furthermore, metal hydrides can store thermal energy indefinitely without insulation.
There are two main reasons for the limited use of metal hydride heat pumps at the present time. One reason is the low efficiency of metal hydride heat pumps compared to vapor compression heat pumps, and the other reason is the reliability problems of metal hydrides after many absorption and desorption cycles. Metal hydride reliability has been significantly improved, and over 100,000 hydriding and dehydriding cycles have been successfully demonstrated for a number of commonly used metal hydrides.
Japanese Patent No. 63-27624 (1988) shows a metal hydride heat pump constructed as a body within which high, medium and low temperature containers enclosing metal hydride oscillate with a rotational motion to transfer heat to a fluid.
Japanese Patent No. 1-21432 (1989) discloses metal hydride within two adjoining containers which are attached but whose volumes are not interconnected. The first embodiment reciprocates linearly along the axis of the containers so that at any one time the two containers are in thermal contact with different heat exchangers, and a second embodiment reciprocates with rotational motion to accomplish the same result.
Each of these devices operates on the same well established principle. A container with higher temperature metal hydride is exposed to heat at an input heat exchanger, and the heat causes the hydride to decompose and pressurize the sealed container with hydrogen, thus storing heat energy within the container. Either an increase of pressure or a decrease in temperature within the container will then cause the hydrogen to be reabsorbed by the metal hydride. The higher temperature metal hydride container is then moved to the mid-temperature heat exchanger where it is exposed to a lower temperature, and the metal hydride is reformed as the hydrogen is absorbed and the container gives up heat to the mid-temperature heat exchanger. The higher temperature metal hydride is then moved back to the input heat exchanger to absorb more heat energy, and at the same time a container with lower temperature metal hydride is moved into contact with the mid-temperature heat exchanger. The lower temperature metal hydride then picks up heat from the mid-temperature heat exchanger and moves it to the lower temperature heat exchanger in the same manner that the higher temperature metal hydride container operates. Furthermore, since the two containers are attached, their movement is simultaneous.
Essentially, a bucket brigade of packages of heat is established, and it should be appreciated that as these packages of heat are moved they not only heat the cooler region, but they also cool, that is remove heat from, the hotter region. Thus, this arrangement can be used as either a heater or air conditioner.
However, the disadvantage of these prior art structures is the low efficiency due to thermal losses during each heating and cooling of the non-reactive containers which enclose the metal hydrides. Furthermore, the energy consumption of these prior art metal hydride heat pumps also increases as the difference between ambient temperature and the hydride generated temperature increases.
Thus, there is still a need for improving efficiency beyond the efficiency offered by the metal hydride heat pumps of the prior art.